1. Field of the Invention
This invention relates to a method and apparatus for improving fluid catalytic cracking (FCC) feed nozzle performance and reliability. More particularly, the invention is directed to a nozzle assembly located near the bottom of the riser reactor for achieving a desirable feed/catalyst contacting condition similar to the modern side entry feed nozzles located at an elevated riser location, but without the need to change the riser, thus substantially reducing the capital cost required to benefit from achieving such a desirable feed/catalyst contact condition.
2. Description of Related Art
As is well known, a typical fluid catalytic cracking (FCC) unit consists of a reactor and a regenerator. Long chain hydrocarbon, such as vacuum gas oil, is injected into the riser reactor through feed nozzles to contact with hot, regenerated catalyst to crack into small, more valuable products, such as gasoline. The catalyst is temporarily deactivated in the riser reactor during the reaction due to the deposition of a coke layer on the surface. The product gas is separated out at the top of the reactor and sent to the product recovery system. Spent catalyst is then transported to the stripper where steam is introduced to recover gas products trapped in the spent catalyst. The stripped spent catalyst is then transported to the regenerator where air is introduced to burn off the coke layer on the surface to reactivate the catalyst. The regenerated catalyst is then transferred back to the riser reactor, which completes the catalyst circulation cycle.
An FCC feed nozzle is critical to unit performance. It determines the uniformity of initial contact of feed with the regenerated catalyst, which has profound effects on dry gas yield and coke selectivity. Most modern FCC feed nozzles are side-entry nozzles that are installed on the periphery of the vertical riser section at an elevated location. This is the preferred option for a grass-root FCC. However, some older FCC units still have feed injection systems located at the bottom of the riser and there could be limitations, such as high capital cost and the pressure-balance of the unit, to convert these units to vertical risers with modern side entry nozzles. The riser, at the location of the bottom injection feed nozzles, may be oriented either horizontally, vertically or inclined.
Applicant's U.S. Pat. No. 5,979,799 discloses a side entry nozzle having a spray pattern that is angled upwardly at an angle of up to 30° from the axis of the nozzle.
Applicant's U.S. Pat. No. 6,387,247B1 discloses a nozzle tip design for a single, centrally located bottom entry nozzle having an upward discharge of from about 10° to about 80° from the axis of the nozzle.
U.S. Pat. No. 4,778,658 to Nielsen discloses a single, centrally located bottom-feed nozzle having angled injection quills for injecting hydrocarbon into an enclosure mixing chamber inside the feed nozzle for better mixing of hydrocarbons and steam, and better atomization prior to injecting the hydrocarbon/steam mixture into the riser for contacting with the catalyst.
U.S. Pat. No. 4,808,383 to Buyan discloses a bottom-feed injection system consisting of multiple feed nozzles with their axis and discharge ends parallel to the axis of the riser.